Make tokio an optional dependency, and add fn ws_receive

[emilk]

This strips a lot of dependencies from the native build by default, to make tokio opt-in.

It also adds a new ws_receive function that is simpler and faster on native, for when you only need to receive message, and never send any.

TODO

• Use document-features crate
• Test with tokio
• Test without tokio
• Test web
• Test with Rerun